The viscosity of lubricating oils is typically dependent on temperature. With an increase in oil temperature, the viscosity of the oil generally decreases; as the temperature of the oil decreases, the viscosity of the oil generally increases. At high temperatures where modern engines typically operate, it is important to maintain viscosity within specified ranges to properly lubricate moving parts of the engine. Additionally, the lubricating oils may be exposed to low temperatures from the environment when the engines are shut off; in these conditions, the viscosity of the oil must be low enough so that the oil will flow when under engine starting conditions. The acceptable oil viscosity ranges for high and low temperatures are specified by the SAE J300 standard.
Lubricating oils also encounter high shear rates while being used in engines. Shear rates as high as 106 s−1 have been reported in literature. The viscosity behavior of lubricants under high temperature, high shear (HTHS) conditions may have an impact on fuel economy. Fluids with relatively high HTHS viscosities typically exhibit poor fuel economy due to the formation of a thicker oil film at the boundary of the engine surfaces. By contrast, fluids with relatively low HTHS viscosities may form a thinner film thickness thereby exhibiting improved fuel economy.
Base oils typically cannot meet the viscosity requirements without the addition of additives such as viscosity index improvers. Viscosity index improvers (VII's) reduce the extent to which the viscosity of lubricants change with temperature, and are used to formulate oils that meet the SAE J300 standard. Suitable Viscosity Index Improvers are polymeric materials that may be derived from ethylene-proplyene copolymers, polymethacrylates, hydrogenated styrene-butadiene copolymers, polyisobutylenes, etc.
Ethylene-propylene copolymers are typically used to provide Viscosity Index Improvers for engine oils. The ethylene content of such copolymers may range from 45 to 85 mole %. Viscosity Index Improvers derived from polymers containing about 60 mole % ethylene are commonly used and require a relatively higher usage or treat rate in oils in order to meet SAE J300 requirements; however, Viscosity Index Improves derived from polymers containing higher than about 65 mole % ethylene to 85 mole % ethylene generally require lower usage or treat rate in oils in order to meet SAE J300 requirements.
The high ethylene polymers, e.g., ethylene contents of 65 mole % to 85 mole % ethylene, are used to improve low temperature properties of lubricating oils. Without being bound by theory, it is believed that at low temperatures, polymers with high ethylene content are thought to undergo intramolecular contraction or folding, leading to a lower viscosity of the oil, when compared to a low ethylene (˜60 mole %) amorphous polymers that do not exhibit such behavior. As a result of such polymer behavior, low temperature properties, such as CCS (cold crank simulator) viscosity, are improved. However, a high ethylene polymer chain may also interact with other chains, or with waxy components that are contained in the oil composition, leading to gel formation. Gel formation is undesirable, as it causes a lack of lubricating oil circulating to the engine, which may lead to engine failures. To prevent the inter-molecular interactions or polymer-waxy component interactions, high levels of pour point depressant may be added to the oil. Higher levels of pour point depressants serve to mitigate but not completely solve the problem.
Accordingly, there is a need to provide viscosity modifiers for lubricant compositions that enable the lubricant compositions to meet the SAE J300 standards while providing lubricant compositions that exhibit improved fuel economy, low temperature properties, and gelation-free behavior.
In accordance with exemplary embodiments, the disclosure provides a lubricating oil composition and methods of operating an internal combustion engine to provide improved engine operational performance. The lubricating composition includes a major amount of oil of lubricating viscosity; a minor amount of at least one olefin copolymer having a number average molecular weight greater than about 10,000 up to about 300,000. The olefin copolymer is grafted with (A) a vinyl-substituted aromatic compound, and (B) a compound selected from the group consisting of a C5-C30 olefin, an internal olefin, a polyalkylene compound, and combinations thereof, wherein a mole ratio of AB in the reaction mixture ranges from about 0.25:1 to about 5:1. The lubricating oil composition may optionally include a minor amount of at least one non-grafted olefin copolymer, styrene-isoprene copolymer, methacrylate copolymer, or styrene butadiene copolymer have number average molecular weight greater than about 50,000 up to about 300,000.
In another exemplary embodiment, the disclosure provides an olefin copolymer viscosity index improver. The olefin copolymer is an extruder reaction product of (a) an olefin copolymer backbone having a number average molecular weight ranging from greater than about 10,000 to about 300,000; and (b) a grafting component including a vinyl-substituted aromatic compound and a grafting component selected from C5-C30 alpha olefins, internal olefins, polyisoalkylenes, and combinations thereof.
Another exemplary embodiment provides an extruded non-dispersant olefin copolymer that is a reaction product of (a) an olefin copolymer, wherein the copolymer has a number average molecular weight ranging from greater than about 10,000 to about 300,000; and (b) a grafting component substantially devoid of carboxylic functionalizing groups including (A) a vinyl-substituted aromatic compound and (B) a component selected from the group consisting of C5-C30 alpha olefins, internal olefins, polyisoalkylenes, and mixtures thereof.
Yet another exemplary embodiment of the disclosure provides a method for improving fuel economy in a vehicle. The method includes lubricating an engine of the vehicle with a lubricant composition including a major amount of oil of lubricating viscosity; and a minor amount of at least one olefin copolymer having a number average molecular weight greater than about 10,000 up to about 300,000. The olefin copolymer is grafted with about 1 to about 30 weight percent of a combination of (A) a vinyl-substituted aromatic compound, and (B) a component selected from a C5-C30 olefin, an internal olefin, a polyisoalkylene, and mixtures thereof. The lubricating oil composition may optionally include a minor amount of at least one non-grafted olefin copolymer, styrene-isoprene copolymer, methacrylate copolymer, or styrene butadiene copolymer have number average molecular weight greater than about 50,000 up to about 300,000.
Accordingly, a primary advantage of the exemplary embodiments may be improved low temperature properties without forming gels at high temperatures. Another advantage may be reduced need for pour point depressants in lubricant compositions containing the grafted olefin copolymers described herein. Yet another advantage of embodiments of the disclosure may be greater fuel economy.